This invention relates to an electrical circuit conforming to the IEC 61158-2 standard with water detection means comprising a physical layer attribute modifier, for use in detecting the ingress of water into field devices in an Intrinsically Safe area.
Fieldbus (or field bus) is the name of a family of industrial computer network protocols used for real-time distributed control, now standardized as IEC 61158. A complex automated industrial system, for example a fuel refinery, usually needs an organized hierarchy of controller systems to function. In this hierarchy there is a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non time critical communications system (e.g. Ethernet). At the bottom of the control chain is the fieldbus, which links the PLCs to the components which actually do the work such as sensors, actuators, electric motors, console lights, switches, valves and contactors.
Fieldbus is often used in Intrinsically Safe environments, for example combustible atmospheres, and in particular gas group classification IIC, Hydrogen and Acetylene, and below, for example gas group IIB and IIA, for gas and/or dust. Using the Fieldbus protocol, field instruments and equipment in such an environment are controlled and monitored remotely via an electrical communications circuit often provided in the same electrical circuit as the power to drive the field instruments.
Fieldbus physical layer diagnostics for IEC 61158 type networks has been introduced successfully to the mainstream processing industry in the last few years. The physical layer specifications are standardised as IEC 61158-2. In a typical electrical communications circuit there is a power supply, an Intrinsic Safety barrier of some kind, a trunk section leading out into the field, and one or more devices, or device couplers with spur sections connected thereto, as the load thereof. The devices send data signals in use to a control system, usually mounted in a non Intrinsically Safe area at the power supply end of the circuit. A diagnostic module is also mounted in the electrical circuit, usually at the same location as the control system, and it works by measuring physical layer attributes of the electrical circuit and the network hardware, and in part, the physical software or protocol being used.
Changes in IEC61158-2 loads and/or physical layer attributes can often lead to a segment failure, and the diagnostic module is set up to detect such changes so that remedial action can be taken.
Water ingress into field devices is one fault that must be detected in order to prevent corrosion of the electrical terminals or cables inside the field device by electrolytic erosion, which can eventually lead to device failures. In extreme cases submersion of the signal lines or terminals in water or other conductive fluids can lead to swift full signal or segment failures.
In any event, instruments in outdoor hazardous areas must be protected to a minimum of IP54 in accordance with the EN60529 standard, and water ingress represents a breach of this. In addition, condensing water vapour inside the instrument enclosure usually infringes the humidity requirements of the instrument, as most specifications allow only 95% relative non-condensing humidity.
Typically, field instruments (devices) have terminal and electronic enclosures. The outer terminal enclosures comprise a small volume, ‘O’ ring sealed enclosures with at least 1 cable gland entry, typically 2 with one blanked off. The electronic enclosures have no cable entry or a need to remove the cover on site, so the likelihood of water ingress is far lower. They are often fitted with potted electronics and/or have ‘O’ ring seals with long threaded covers, and are often able to meet ratings of IP66 to IP68. Unfortunately ingress of water into the terminal enclosures is a common occurrence in practice.
For every segment of the system, there are typically up to 16 instruments. This means that the probability of the terminal enclosure of at least one instrument on a segment filling with water is very high in comparison to the device coupler housing. These need less maintenance and have a larger enclosure volumes with drain facilities.
One known way to detect water ingress in a field device is to detect a ground fault using an earth leakage detection system, and such an arrangement is shown in FIG. 1. In this arrangement an electrical circuit 1 comprises an earth leakage detection system 2, which detects conductivity 3 in a device 4 between the terminals/poles 5 and the shield 6, and therefore the ground 7, through conductive liquid 8. This takes place inside the device enclosure 9, adjacent to the device electronics 10.
Measuring for an earth fault is advantageous over measuring for a differential change because it is discernable over load changes. A load change and the effects of water bridging signal poles 5 would cause similar differential changes in the circuit 1, so a purely differential measurement would not be reliable.
However, these systems have a number of drawbacks. First of all, they can only detect the presence of high conductivity water. Cable length and voltage restrictions to meet the requirements of Intrinsic Safety make the detection of purer water impossible. The applicant, as well as other companies like MTL (Cooper Crouse-Hinds Co) produce such systems, and as an example the MTL 4220 product can only detect conductivity equivalents of 10 kOhm or less, whereas measurements of 250 kOhms to 1 MOhm and higher are typical for clean rain water and precipitation at low excitation detection voltages.
Even complete immersion of the terminals 5 in condensate or rain water with a very low conductivity will not lead to a normally measurable current or impedance change. However such an event will start the process of galvanic corrosion which can cause a failure. Bench testing has shown a low concentration saline solution will dissolve (by galvanic corrosion of the anode) the signal wires within a matter of minutes. Dissolved metal trace elements leaking form the terminals and wire can quickly increase the conductivity of the water so it can be detected, but by then failure can occur before any evasive action can be taken.
The second main problem is that even if the water is conductive enough to be detected straight away, the poles 5 are already submerged when detection is made. Submersion can rapidly lead to damage and signal problems, often before remedial action can be taken. An earth leakage detection system like that shown in FIG. 1 relies on the electrolytic affect between the terminals 5 and the shield 6 or ground 7, and at this point the damage is already in progress. If the conductivity equivalent is 10 kOhms or less, the acceleration in conductivity due to localised metals dissolving into the solution can quickly result in an over current situation, and too rapidly to allow for proactive maintenance. In other words, the principal of monitoring for the submergence of terminals is a flawed one in any event, because damage in some cases may be unpreventable.
Another drawback with earth leakage detection systems like that shown in FIG. 1 is that they are autonomous systems that comprise hardware and cabling for sensing, alarm and power, which is additional to the existing electrical circuit 1. This adds costs and increases the likelihood of failures. The power supply must also be rendered Intrinsically Safe.
A further drawback with earth leakage detection systems is that they rely on connection to the shield 6, which often does not extend inside the field device 4, or is not present in a suitable position for connection.
An alternative known way to detect water ingress in a field device is to use an autonomous sensor inside the field device which can detect the presence of water, and such an arrangement is shown in FIG. 2. In this arrangement an electrical circuit 20 comprises an autonomous detector and transmitter 21 disposed inside the terminal enclosure 22 of the field device 23, which detects the onset of water ingress 24 by sensing conductivity 25 with detector probes 26. An alarm is sent through separate power and signal lines 27 to a diagnostic and alarm module 28.
Because this system is autonomous it can be adapted to detect high purity low conductivity water, and by positioning the probes 26 away from the terminals/poles 29 in the field device 23 it can do so long before any damage is caused to the terminals/poles 29 or the device electronics 30. There is also no requirement for any connection to the shield. Therefore, this arrangement has distinct advantages over the earth leakage detection system shown in FIG. 1.
However, there are still a number of major drawbacks. Firstly, given the number of field devices used in practice on site, it is very complex and very costly to install such autonomous systems in each and every field device.
In addition, this solution still requires hardware, electronics, cabling and a power supply additional to the existing infrastructure, all of which must be rendered Intrinsically Safe, which requires calculations for all parameters and additional wiring diagrams, which all adds to the costs.
This additional equipment increases the likelihood of failure generally. In addition, each field device will need an additional cable gland, which actually increases the likelihood of water ingress.
Therefore, a solution is required which can detect low conductivity water ingress in a field device, detect water ingress before any terminal in the field device is wetted, and relay that information to maintenance crews without the need for additional wiring or a wireless link, and via long trunk cables and any isolated device couplers. This eliminates the requirement for any additional power or other wiring, other than simple connection to a bus (including a shield if required). In order to be practical the solution must also meet the requirements of the Intrinsic Safety (EEx ia or ib) “simple apparatus” definition, and have connection means suitable for Intrinsically Safe wiring practice. Some field device terminal enclosures are very small in area and/or in volume, so any solution must also be small and simple. Finally the solution must maintain the integrity of the Fieldbus signal.
It would be theoretically possible to achieve many of the above goals by making water detection a function of the field device. In other words, the field device electronics can include a water detection function, and the field device electronics can communicate the detection of water to the control system via its Fieldbus telegram. However, such an arrangement would be self-defeating because it would essentially comprise an entire Fieldbus field device installation on its own. In addition, integrating such a function into hundreds of different Fieldbus device types would be very costly from a design and manufacturing point of view, and not all manufacturers would be willing to integrate such systems into every model type. In addition, the function would consume part of the Fieldbus telegram, limiting the communication potential of the primary function of the device.